Schlieren technique is widely used in the visualization and measurement of combustion fields due to its high sensitivity. The data obtained by single-direction schlieren imaging is limited. With the improvement of measurement requirements, schlieren computerized tomography (CT) has become an important research direction. Schlieren CT reconstructs the test field from multidirectional projections. Due to inevitable system installation errors, the projections will be imaged at different positions on the sensors with different light intensities. In this paper, a parallel transmission schlieren CT system is designed with six directions. In order to solve the above problems, firstly, a multidirectional calibration method of schlieren CT based on affine projection is proposed to determine the internal and external parameters of the system. Then, the calibrated schlieren technique is used to establish the quantitative relationship between the grayscale values of the multidirectional schlieren images and the light displacements. With the calibration results, projections can be remapped to a unified coordinate system to reconstruct the three-dimensional distributions of the flow field.
In the background-oriented schlieren (BOS), the 3D deflection caused by refractive index gradient is projected onto the camera imaging plane with one dimension leaved out, and sensed as a 2D displacement using the images of background with and without the flow field. In the reconstruction, instead of decomposing the 2D displacement as in previous studies, we project the elements in the BOS weight matrix to 2D. By doing so, this technique reduces memory usage and improves the reconstruction time as the number of rows in the weight matrix is decreased by a third, and the errors produced in 2D displacement decomposition are avoided.
The measurement and visualization of transient three-dimensional (3-D) physical parameters (density and temperature) distribution of complex flow fields are critical technologies for the characteristics studies of flow fields in modern energy engineering. Among the optical computed tomography (OCT) methods, Moiré tomography has the advantages of simple optical path structure, strong anti-interference ability and wide measurement range, which is especially suitable for complex flow field measurement in noisy environments. Acquiring the transient phase information from the moiré projection is of great importance for the dynamic 3-D parameters reconstruction of complex flow fields. In this paper, the dynamic phase retrieve methods including Fourier and spatial phase-shifting in moiré tomographic are studied, respectively. In the Fourier method, an adaptive first-order spectrum extraction algorithm for Fourier transform moiré fringe and a phase calculation method are proposed. Through this, the projection phase can be obtained directly by multiplying the inverse Fourier transform of the positive first-order spectrum of deformed fringe with the inverse Fourier transform of the negative first-order spectrum of reference fringe. In spatial phase-shifting method, a spatial phase-shifting- interferometry-based moiré volume computed tomography (MVCT) method was proposed to extract the radial shearing phase distribution of grid moiré fringe. The measured results for the first-order partial derivative of the phase projection of a propane flame both by Fourier and spatial phase-shifting methods in the experimental moiré computed tomography systems are presented. The research will be valuable for monitoring the combustion state in energy engineering.
Background-oriented schlieren (BOS) is a novel flow visualization technology, which has the advantages of simple device, low cost, and quantitative measurement. In this paper, a BOS technique with two-dimensional color coding background is presented, which can be referred to as color background-oriented schlieren (CBOS). Different from traditional background schlieren technique, we designed a pattern combining ‘cosine dot’ and two-dimensional color coding as the background of BOS technique. Inspired by the coding principle based on De Bruijn sequence coding in the spatial coding method of traditional structured light coding technology, the pattern of ‘cosine dot’ is generated by multiplying the fringes in two perpendicular directions. In order to integrate the intensity value of cosine dots and corresponding encoded colors into a single pattern, and to separate them completely, the HSV color model is applied to address the problem. We calculate the perturbation degree of test flow fields by searching positions of the local intensity maximum of the cosine dot patterns with and without flow fields. The function of two-dimensional color coding is employed to locate the relative position of each local intensity maximum point. Finally, we carried on the simulation experiment and the real experiment to verify the feasibility of this method.
In automatic camera calibration with traditional chessboard patterns, the corner sorting results are usually influenced by the rotation angle of the calibration pattern. Therefore, this paper designs an improved chessboard pattern and corresponding corner extraction algorithm. In the new pattern, five nested rings are added near the four corners to determine the sorting origin.After completing the checkerboard positioning process, the improved algorithm first identifies the nested contours by five concentric circles on the four corners of the calibration plate. Then it identifies the center of each contour and determines the order of four centers. After that it uses a perspective projection to correct part of the image on the calibration plate and reorder the calibration points in order to obtain a sequence of calibration points. Experiment results show that the corner detection results are correct and the new method has a lower mean reprojection error per image.
Background oriented schlieren(BOS) technology is an efficient measurement method for quantitative diagnostics of fluid field in recent decades, and it has a broad application prospect in flow field measurement. It not only has high spatial and temporal resolution, as well as can be employed for quantitative measurement of the density gradient distributions of convection fields. In this paper, a new method for reconstructing density distributions is proposed. Initially, we obtained the point displacement image according to the basic principle of BOS. Secondly we used the local basis function method to discretize the volume to obtain the coefficient matrix. We had to choose an appropriate finite support basis function to ensure the coefficient matrix was sparse. Finally we obtained the density field by using algebraic iteration method and other methods.Numerical simulation experiments are presented to verify our method. The experimental results of refractive index and density field distribution of flow field are obtained after the simulation experiment, which indicates that the method of CTBOS technology can obtain the quantitative refractive index and density distribution of flow field, while in the real condition, there is great application value.
Fourier transform profilometry method has great value in high-speed three-dimensional shape measurement. In the method of Fourier transform profilometry, it is necessary to obtain the phase of the deformation fringe containing the height information of the object through Fourier transform, frequency domain filtering and inverse Fourier transform. Filtering in frequency domain is a very important and essential process. Filtering window is usually selected manually, which is inefficient and subjective. Too large filtering window can not filter useless information, and too small filtering window will lose the height information of the object. In this paper, an adaptive spectrum extraction method is used. In order to be more convenient and simple, this paper presents a method of frequency domain filtering based on convolution neural network.Convolution neural network can realize image recognition and image feature extraction. The proposed method uses convolution neural network to identify the carrier frequency components carrying the details of the object in the spectrum image. This paper introduces the theoretical analysis and the training process of convolution neural network. The adaptive spectrum extraction method and the convolution neural network method are compared. The method of spectrum extraction based on convolution neural network is feasible.
Background Oriented Schlieren (BOS) technique can be applied for quantitative flow field diagnosis with simple experimental configurations. One of the crucial steps of BOS techniques is the extraction of image displacement vectors. The cross-correlation algorithm widely used in PIV techniques have been introduced to address the BOS extraction. However, the cross-correlation algorithm depends on interrogation windows, which usually results in low spatial resolving or unstable results. This paper proposes an improved BOS approach based on three step phase-shifting algorithm with the usage of a colored-fringe pattern as background. RGB coded carrier-fringe image is composed of three phase-shifted images. Displacement vectors are extracted by comparing the different phases between the corresponding points in three separated images. This technique avoids the problem of selecting the interrogation window. Only one image is required in this approach. An experimental setup on the measurement of hot air gun was carried out by use of our proposed method. The results demonstrate that this technique can be used for quantitative measurement in flow field.
Synthetic aperture particle image velocimetry (SAPIV) is a flow field diagnostic technique that provides instantaneous velocimetry information non-intrusively. In SAPIV, particle scattering images are captured from different cameras with camera array configuration. To acquire refocusing images, images are remapped and accumulated in pre-designed remapping planes. During the refocused images, particles that lie in the remapped plane are aligned and appear sharp, whereas particles off this surface are blurred due to parallax between the cameras. During the remapping process, captured images are back-projected to different remapped planes of different depth z within the volume. The projected images from different cameras, which are called remapped images, are merged to generate refocused images at different depth z. We developed a remap method based on the weight coefficient to improve the quality of the reconstructed velocity field. The images captured from the cameras are remapped into different remapped planes by use of homography matrix. The corresponding pixels of the remapped images in the same remapped plane are first added and averaged. The corresponding pixels of the remapped images in the same remapped plane are multiplied and the obtained intensity values act as the weight coefficients of the intensity in the added refocused image stacks. The unfocused speckles can be restrained to a great degree, and the focused particles are retained in the added refocused image stacks. A 16-camera array and a vortex ring field at two adjacent frames are simulated to evaluate the performance of our proposed method. In the simulation, a vortex ring can be clearly seen. An experimental system consisting of 16 cameras was also used to show the capability of our improved remap method. The results show that the proposed method can effectively restrain the unfocused speckles and reconstruct the velocity field in the flow field.
In 3D particle image velocimetry (PIV), when laser transports through dense trace particle field, scattering light intensity vary in different directions. In this article, we build 5 fields of different densities, and each field contains one vortex ring. The diameter of the vortex ring is 2mm, and the particles are dense in the ring and sparse outside the ring. Based on the Mie scattering theory and Monte Carlo method, we compute the laser intensity difference along the direction of incident light in each particle volume when the laser beam transports through it, and obtain the relationship of laser intensity, particle density and the distance of laser transportation. The variation of laser intensity could also be viewed from different directions. We also discussed the influence of light intensity variation on integrated imaging particle-imagevelocimetry (PIV) image’s quality in this paper. To deal with this variation, we propose a new light intensity equalized compensation method. By using this method, we can reduce the influence of attenuation when laser light transports through dense particle areas. During the simulation process, a camera array is set to detect the forward and back direction of the laser beam in the region, and the light intensity is recorded by different pixels. Light intensity attenuation of different positions is considered. All cameras are treated as pinhole models. The results show that front scattering and back scattering have great effects on integrated imaging PIV. The compensation method is used in experiment to preprocess particle images.
The principle of spatial filtering experiment has been introduced, and the computer simulation platform with graphical user interface (GUI) has been made out in Matlab environment. Using it various filtering processes for different input image or different filtering purpose will be completed accurately, and filtering effect can be observed clearly with adjusting experimental parameters. The physical nature of the optical spatial filtering can be showed vividly, and so experimental teaching effect will be promoted.
Optical theories were all originating from the experimental phenomena, as a result, we can combine the theories and experiments organically in optics teaching that can make the teaching content more intuitive and vivid to stimulate the students' learning interests. In this paper, we proposed the "Experiment-Guidance-Theory" teaching mode in optics course by integrating the theory of optics courses with corresponding experiments. Before the theoretical learning, the students would do some basic experiments to observe the optical phenomena on themselves and answer the corresponding illuminating questions to put themselves into the role, and then the teachers explain the corresponding optical methods and theories, at last, the students must attend an expansive discussion and innovation experiment around the optical theme to expand their scientific view and innovation ability. This is a kind of inquiry-based teaching method, which can stimulate the students' studying interests and improve learning initiative. Meanwhile, the ideas of scientific research also be integrated into teaching, which is beneficial to cultivate students' ability to carry out innovative research.
Python is a popular open-source programming language that can be used to simulate various optical phenomena. We have developed a suite of programs to help teach the course of laser principle. The complicated transverse modes of the symmetric confocal resonator can be visualized in personal computers, which is significant to help the students understand the pattern distribution of laser resonator.
Flame tomography of chemiluminescence is a necessary combustion diagnostic technique that provides instantaneous
3D information on flame structure and excited species concentrations. However, in most research, the simplification of
calculation model of weight coefficient based on lens imaging theory always causes information missing, which
influences the result of further reconstructions. In this work, an improved calculation model is presented to determine
the weight coefficient by the intersection areas of the blurry circle with the square pixels, which is more appropriate to
the practical imaging process. The numerical simulation quantitatively evaluates the performance of the improved
calculation method. Furthermore, a flame chemiluminescence tomography system consisting of 12 cameras was
established to reconstruct 3D structure of instantaneous non-axisymmetric propane flame. Both numerical simulating
estimations and experiments illustrate the feasibility of the improved calculation model in combustion diagnostic.
Tomographic particle image velocimetry (Tomo-PIV) is a new developed technique for three-component threedimensional (3C-3D) velocity measurement of the flow field based on the optical tomographic reconstruction method, and has been received extensive attention of the related industries. Three-dimensional light source illuminating the tracer particles of flow field is a critical application for tomographic particle image velocimetry. Three-dimensional light source not only determines the size of measurement volume and the range of the scope of application, but also has a great influence on the image quality. In this work, we propose a rectangular light amplification system using powell lens, prisms and two reflectors. The system can be optimized if given the system parameters based on the theoretical model. The rectangular light amplification system will be verified experimentally by measuring the cross section size of the illuminated light source. A 60mm×25mm cross section of rectangular three-dimensional light source can be obtained by using the rectangular light amplification system. The experiments demonstrate the the feasibility the proposed system.
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